1. Field of the Invention
This invention relates to refrigerated superconducting MR magnets having integrated cryogenic gradient coils. Such structures of this type generally reduce the size and weight and therefore, the cost of the superconducting magnet required for an MR imaging system while substantially eliminating the resultant eddy currents produced in the magnet.
2. Description of the Related Art
Present superconducting MR magnets employ windings which operate in liquid helium to maintain the temperature at approximately 4 K. The liquid helium pool requires a vessel which is vacuum tight and which meets ASME pressure vessel requirements; such a vessel is typically made of welded aluminum alloy cylinders and flanges. Thermal radiation shields, of which two are typically used, are also made of welded aluminum pieces and contain the helium vessel. When the gradient coils are pulsed in the bore of the magnet, the resulting time changing magnetic flux in any of the electrically conducting cylinders sets up eddy currents which, in turn, produce other magnetic fields which degrade the quality of the desired gradient field in space and time. This behavior makes it attractive for the aggressive pulse sequences which are routinely used in MR imaging today to use a second set of gradient coils in the magnet bore. This shield gradient coil sets up fields which counteract those of the main gradient coil radially outside of the shield coil, thus greatly reducing any mutual inductance with conducting members such as the thermal shields and minimizing the resultant eddy currents.
The use of such shield gradient coils increases the radial thickness of the gradient coil set relative to a simple single coil set because of the required gap between them and dictates the size, and thus weight and cost, of a conventional magnet, which normally lies radially outside of the shield gradient. Therefore, a more advantageous system, then, would be realized if the magnet could be made to have no mutual coupling to the gradient coil set and if it could be placed in much closer proximity to the gradients without having any deleterious eddy currents induced in it. The resulting system could be dramatically smaller and less expensive than existing ones.
It is apparent from the above that there exists a need in the art for a refrigerated superconducting MR magnet which is reduced in size, weight and cost through simplicity of parts and uniqueness of structure, and which at least equals the imaging characteristics of known superconducting magnets, but which at the same time substantially reduces the amounts of resultant eddy currents produced in the magnet by the gradient coils. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.